Let radius of each cone = r
Ratio between their slant heights = 5:4
Let slant height of the first cone = 5x
and slant height of second cone = 4x
Therefore, curved surface area of the first cone =
πrℓ = πr × (5x) = 5πrx
curved surface area of the second cone = πrℓ = πr × (4x) = 4πrx
Hence, ratio between them = 5πrx : 4πrx = 5 : 4